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Abstract

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​Multi-effect distillation (MED) systems are proven
and energy efficient thermally-driven desalination systems for handling
harsh seawater feed in the Gulf region. The high cycle efficiency is
markedly achieved by latent energy re-use with minimal stage
temperature-difference across the condensing steam and the evaporating
saline seawater in each stage. The efficacies of MED system are (i) its
low stage-temperature-difference between top brine temperature (TBT) and
final condensing temperature, (ii) its robustness to varying salinity
and ability to handle harmful algae Blooming (HABs) and (iii) its
compact foot-print per unit water output. The practical TBT of MED
systems, hitherto, is around 65 °C for controllable scaling and fouling
with the ambient-limited final condenser temperature, usually from 30 to
45 °C.

The adsorption (ADC) cycles utilize
low-temperature heat sources (typically below 90 °C) to produce useful
cooling power and potable water. Hybridizing MED with AD cycles, they
synergistically improve the water production rates at the same energy
input whilst the AD cycle is driven by the recovered waste heat. We
present a practical AD + MED combination that can be retrofitted to
existing MEDs: The cooling energy of AD cycle through the water vapor
uptake by the adsorbent is recycled internally, providing lower
temperature condensing environment in the effects whilst the final
condensing temperature of MED is as low as 5–10 °C, which is below
ambient. The increase in the temperature difference between TBT and
final condensing temperature accommodates additional MED stages. A
detailed numerical model is presented to capture the transient behaviors
of heat and mass interactions in the combined AD + MED cycles and the
results are presented in terms of key variables. It is observed that the
water production rates of the combined cycle increase to give a GOR of
8.8 from an initial value of 5.9.